1. Field of the Invention
The present invention relates to a surface acoustic wave device and, more specifically, relates to a surface acoustic wave device in which short-circuit breakages are unlikely to occur between electrode fingers of an IDT electrode of different potentials even when the pitch between the electrode fingers is small and in which reliable electrical connections are realized between the IDT electrode and a wiring electrode.
2. Description of the Related Art
Surface acoustic wave devices are widely used as filters in mobile communication devices and so forth.
In a surface acoustic wave device, a relationship v=f×λ exists, where v is the acoustic velocity of a surface acoustic wave, f is the frequency of the surface acoustic wave and λ is the wavelength of the surface acoustic wave. Therefore, in the case where the acoustic velocity of the surface acoustic wave, which is determined by various conditions such as the material of a piezoelectric substrate, is constant, there is an inversely proportional relationship between the frequency of the surface acoustic wave and the wavelength of the surface acoustic wave, in other words, the pitch of the electrode fingers of the IDT electrode.
In recent years, the frequencies used in mobile communications have been increasing, and in conjunction with this, it has been necessary to reduce the pitch of the electrode fingers of IDT electrodes. However, when the pitch of the electrode fingers of IDT electrodes is reduced, the insulation resistance between electrode fingers of different potentials becomes smaller and therefore there is a risk of insulation breakages occurring due to short circuits between the electrode fingers of different potentials when a surge voltage is applied to the surface acoustic wave device.
Generally, insulation breakages between electrode fingers of an IDT electrode of different potentials are more likely to occur along a path that extends through a piezoelectric substrate than along a path that extends through air. This is because the insulation resistance of a piezoelectric substrate is smaller than that of air.
Accordingly, in a surface acoustic wave device disclosed in Japanese Unexamined Patent Application Publication No. 2001-85968, for example, a method is employed in which a small line width (WS) is adopted as the width of the electrode fingers of an IDT electrode in parts of the electrode fingers that contact the piezoelectric substrate and a large line width (WB) is adopted as the width of the electrode fingers in other parts of electrode fingers in order to ensure that insulation breakages due to short circuits do not occur between electrode fingers of different potentials because of a surge voltage or the like even when the pitch of the electrode fingers of the IDT electrode has been made small.
FIG. 8 illustrates a cross section taken along X-X in FIG. 7 for the case where the method of preventing insulation breakages between electrode fingers of different potentials disclosed in Japanese Unexamined Patent Application Publication No. 2001-85968 is applied to a surface acoustic wave device 200 (refer to FIG. 7) disclosed in Japanese Unexamined Patent Application Publication No. 2005-102158.
Electrode fingers 103a and 103b of IDT electrodes 102 are each formed of two layers, namely, an electrode finger lower layer 103f that contacts a piezoelectric substrate 101 and an electrode finger upper layer 103s that is provided on the electrode finger lower layer 103f. The electrode finger lower layer 103f is formed to have a small line width WS and the electrode finger upper layer 103s is formed to have a large line width WB. This type of structure made up of the electrode finger lower layer 103f and the electrode finger upper layer 103s can be easily formed by using a dry etching method.
With the method disclosed in Japanese Unexamined Patent Application Publication No. 2001-85968, a large insulation resistance can be maintained between the electrode fingers 103a and 103b of different potentials even when a pitch P of the electrode fingers of the IDT electrodes has been made small in order to handle high frequencies.
In other words, when the pitch P is made small, an interval Ds between the electrode finger upper layers 103s of the electrode fingers 103a and 103b of different potentials becomes smaller. However, since air, which has a large insulation resistance, exists in the interval Ds, insulation breakages due to short circuits are unlikely to occur in these parts even when a surge voltage is applied.
On the other hand, an interval Df between the electrode finger lower layers 103f of different potentials can be left at the same size even when the pitch P is made smaller. Although this part of the electrode fingers contacts the piezoelectric substrate 101, which has a small insulation resistance compared to air, since the interval Df does not become smaller, insulation breakages due to a short circuit occurring via the piezoelectric substrate 101 are unlikely to happen even when a surge voltage is applied.
Thus, by employing the method disclosed in Japanese Unexamined Patent Application Publication No. 2001-85968, a large insulation resistance can be maintained between the electrode fingers 103a and 103b of different potentials even when the pitch P of the electrode fingers of the IDT electrode has been made small in order to handle high frequencies.
However, when the above-described method is employed, there is a problem in that an electrical connection between the IDT electrode and a wiring electrode may be inadequate. This will be described below.
A wiring electrode may be provided in a surface acoustic wave device as disclosed in Japanese Unexamined Patent Application Publication No. 2005-102158. A surface acoustic wave device 200 of the related art is illustrated in FIG. 7.
IDT electrodes 102 are formed on a piezoelectric substrate 101. Each IDT electrode 102 includes a set consisting of comb-tooth-shaped electrode fingers 103a and 103b, which have different potentials, and busbars 104. A wiring electrode 105 is connected to each busbar 104. A state is illustrated in FIG. 9 in which the electrical connection between the IDT electrode 102 (busbar 104) and the wiring electrode 105 is inadequate in the case where the method disclosed in Japanese Unexamined Patent Application Publication No. 2001-85968 has been applied to the surface acoustic wave device 200 disclosed in Japanese Unexamined Patent Application Publication No. 2005-102158 illustrated in FIG. 7. FIG. 9 illustrates a cross section taken along Y-Y in FIG. 7.
When the electrode fingers 103a and 103b of the IDT electrode 102 are formed such that the line width WS of the electrode finger lower layer 103f is small and the line width WB of the electrode finger upper layer 103s is large (refer to FIG. 8), as illustrated in FIG. 9, the busbar 104 also has a two layer structure made up of a busbar lower layer 104f and a busbar upper layer 104s, and the busbar lower layer 104f is structured so as to be recessed from the busbar upper layer 104s at an outer peripheral edge of the busbar 104.
Typically, the wiring electrode 105 is often formed using a liftoff method. In other words, the wiring electrode 105 is often formed by arranging a resist having a desired pattern shape and then depositing (vapor depositing) a metal film. However, in the case where the wiring electrode 105 is formed using a liftoff method, when the busbar lower layer 104f is formed so as to be recessed from the busbar upper layer 104s at the outer peripheral edge of the busbar 104, the electrical connection between the busbar 104 and the wiring electrode 105 may be inadequate.
That is, when the wiring electrode 105 is formed using a liftoff method, the portion of the wiring electrode 105 on the piezoelectric substrate 101 is deposited in such a manner as to remain unconnected to the busbar lower layer 104f at the outer peripheral edge of the busbar 104 at the very start of the deposition process. This is because the deposition is started in a state where the busbar lower layer 104f is formed so as to be recessed and the busbar upper layer 104s acts as an overhanging portion.
As the deposition progresses, the wiring electrode 105 is also deposited on the busbar 104, but an end surface of the portion of the wiring electrode 105 on the busbar 104 also grows in a horizontal direction, forms an overhanging portion and covers an end surface of the portion of the wiring electrode 105 on the piezoelectric substrate 101. As a result, the end surface of the portion of the wiring electrode 105 on the piezoelectric substrate 101 is formed with a tapered shape that is inclined in a direction away from the busbar 104 as the deposition progresses.
The deposition is completed with the portion of the wiring electrode 105 on the piezoelectric substrate 101 remaining unconnected to the busbar 104 and remaining unconnected to the portion of the wiring electrode 105 deposited on the busbar 104. In other words, a gap G is formed between the portion of the wiring electrode 105 on the piezoelectric substrate 101, the busbar 104 and the portion of the wiring electrode 105 on the busbar 104.
Consequently, the electrical connection between the IDT electrode 102 (busbar 104) and the wiring electrode 105 may be inadequate. For example, the IDT electrode 102 and the wiring electrode 105 may be completely disconnected from each other. In addition, even if the IDT electrode 102 and the wiring electrode 105 are not completely disconnected from each other, the gap G may be partially formed between the IDT electrode 102 and the wiring electrode 105, thereby increasing the wiring resistance.